A surface acoustic wave device (SAW device) is capable of exciting an acoustic wave by applying power to comb electrodes of an interdigital transducer (IDT) formed on a piezoelectric substrate. The SAW device is widely used for various circuits that process radio signals in a frequency band of, for example, 45 MHz to 2 GHz. Examples of these circuits are bandpass filters for transmission, bandpass filters for reception, local oscillation filters, antenna duplexers, intermediate frequency filters, and FM modulators.
The SAW device needs a cavity above a functional part (electrode fingers of IDT) of a SAW element composed of comb electrodes in order to secure the performance of the SAW element. A conventional SAW device employs a ceramic package having a recess in which the SAW element is mounted. A metal lid covers the recess, so that a cavity can be defined above the functional part of the SAW element. Wire bonding is used to make electrical connections between the SAW element and interconnections provided on the ceramic package.
However, wire bonding uses wires, which prevents downsizing of the SAW device. In order to achieve downsizing, flip-chip bonding is developed. Flip-chip bonding realizes downsizing because of wireless mounting.
Recently, there has been a more strict demand for downsizing the SAW device. There are some cases where even flip-chip bonding is not capable of realizing required downsizing. There is a proposal for further downsizing. Japanese Patent Application Publication No. 2000-261284 describes a SAW element provided on a surface of a piezoelectric substrate. A cover is provided on the surface of the piezoelectric substrate in order to define a cavity above the functional part of the SAW element. The cover is used as a package. This type of packaging is called wafer level package (WLP). This realizes further miniaturization.
The SAW device using the piezoelectric substrate has a problem such that the frequency characteristic is affected due to expansion and contraction of the piezoelectric substrate. The frequency characteristic of the SAW device is more greatly affected as the piezoelectric substrate has a greater amount of expansion and contraction. The piezoelectric substrate expands and contracts with the amount of expansion and contraction that depends on the linear expansion coefficient due to temperature change. The piezoelectric substrate may be typically made of lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), which has a great electro-mechanical coupling coefficient. However, these substrates have great linear expansion coefficients, and greatly expand and contract due to temperature change.